Shader "Retroed"
{
    Properties
    {
        _MainTex("Texture", 2D) = "white" {}
        _ScreenWidth("screen width", float) = 320.0
        _ScreenHeight("screen height", float) = 240.0
        _CellSizeX("size of x cell", float) = 4.0
        _CellSizeY("size of y cell", float) = 4.0
    }
    SubShader
    {
        // No culling or depth
        Cull Off ZWrite Off ZTest Always

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            v2f vert(appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                return o;
            }

            sampler2D _MainTex;
            float _ScreenWidth;
            float _ScreenHeight;
            float _CellSizeX;
            float _CellSizeY;

            fixed4 frag(v2f i) : SV_Target
            {
                float2 uv = i.uv;

                float pixelX = _ScreenWidth / _CellSizeX;
                float pixelY = _ScreenHeight / _CellSizeY;

                return tex2D(_MainTex, float2(floor(pixelX * uv.x) / pixelX, floor(pixelY * uv.y) / pixelY));
            }
            ENDCG
        }
    }
}